Mine backfill process



United States Patent 3,508,407 MINE BACKFILL PROCESS Robert Ben Booth, Stamford, Conn., assignor to American Cyanamid Company, Stamford, Comm, a corporation of Maine N0 Drawing. Filed Mar. 4, 1968, Ser. No. 709,906 Int. Cl. F011;; F021) 13/10, 43/00, 47/00 U.S. Cl. 6135 Claims ABSTRACT OF THE DISCLOSURE A process for filling subterranean cavities by first charging into a cavity a high density water suspension of particulate mineral matter such as ore flotation tailings, sand, slag, which contains a water-reactive setting agent and a high molecular weight water-soluble polymer as a fines retaining agent. The water drains from the mixture after placement without simultaneous exudation of finely divided solids such as the setting agent. The remaining solid material sets to form a cohesive, stabilized load supporting structure.

This invention relates to a process for filling in manmade or natural subterranean cavities. More particularly, this invention relates to a process for filling mined-out areas with particulate ore material and a setting agent in a manner such that the setting agent and the particulate solids are maintained in a homogeneous admixture which is stable and cohesive after setting takes place.

In present mining operations, it is common practice to refill mined-out areas which have been depleted of ore, with solid material comprising sand, crushed slag, and ore flotation tailings previously obtained as a by-product from the ore concentration process. The practice of filling such openings both underground or in surface operations by placement of water suspensions of high bulk density is often referred to as backfilling, and the materials used for such purposes are known as backfill. It is highly desirable to refill the mined-out areas in a manner to provide as much strength as possible thereto. This provides the advantages of preventing or minimizing caving and subsidence of the strata adjacent to and above the mined-out area and permits mining of ore adjacent to the previously mined-out area after the solid material has set.

Particle size distribution in the backfill is an important factor in its efiiciency, since this factor will determine the support properties, percolation characteristics and the density of the backfill. Excessive concentration of fines or colloidal particles reduces the permeability and load bearing capacity. Furthermore, the fine or slime fractions, either in the fill or the added setting agent, tend to drain from the mass after placement, clog mine drainage, interfere with ore haulage, cause wear in the bearings of haulage and mining equipment, and create a serious and costly maintenance problem in cleanout operations. It is common practice to remove a portion of the slimes from the ore tailings or to deslime the tailings as completely as practically possible and to use the coarser portions of the tailings as backfill. This practice has alleviated the drainage and load problems only to a minor degree.

To improve the cohesion and the stability of the backfill mass, the ore tailings and other fill materials usually are mixed with water and a water-reactive setting agent such as Portland cement before or during the placement of the fill. In such operations, water drains from the solid material after placement to permit setting to take place. Unfortunately, in present operations, the draining water carries away fine particles including a large portion of the cement previously added to the fill. This causes a reduction of the strength of the backfill or can even retard the setting of the backfill. The excessive drainage of cement fines therefore necessitates employing an excess of cement to ensure a hardened backfill mass having adequate strength.

An allied problem is the segregation of mineral fines and particularly the fine cement particles as a supernatant layer at the top of the backfill mass after water has drained from the mass. This slime layer is commonly 0.5 to 2 feet deep, remains soft and mud-like, and has poor load bearing qualities. Frequently, this slime layer is flushed off the top of the backfill mass, thus creating a serious disposal problem in underground mining operations.

The hardened backfills obtained by known processes are characterized by an undesirable heterogenous size distribution of solids throughout the backfill mass. This is believed to be caused by the segregation of the coarser portions of the backfill mass after or during placement. The strength of the resultant set backfill is reduced over that which could be obtained with a backfill having the setting agent and fines homogeneously distributed therethrough. Thus, there exists a need to provide a process for filling cavities in the earths crust, particularly minedout areas, with solid material to provide strong support without employing excessive amounts of expensive setting materials.

Accordingly, it is an object of the present invention to provide a process for filling subterranean cavities with set solid material obtained by setting particulate mineral solids such as ore tailings, sand, slags and the like. It is a further object of the present invention to provide a process for filling such cavities, whereby drainage of solid mineral fines and cement or other water-reactive setting agents from the solid particulate filling material is minimized. It is a further object of the present invention to provide a process for filling such cavities without employing excessive amounts of setting material. Further objects of the present invention will be evident from the following detailed disclosure.

In accordance with the present invention, a cavity in the earths crust is filled with a particulate material such as ore tailings and like filling material containing water, a water-reactive setting agent and a water-soluble high molecular weight fines retention agent comprising a water-soluble high molecular weight polymer. This composition can be charged into the cavity by gravity or by pumping. After the placement operation water drains from the solids to permit the solids to set within the cavity.

The use of the water-soluble high molecular weight polymers as fines retention agents in the fill composition provides substantial advantages over the presently employed processes. The polymer promotes substantially complete and very rapid drainage of water from the backfill mass. With a fill composition containing a high molecular weight polymer, the drainage of ore slimes and/or cement fines from the fill can be reduced by as much as about -90%. Furthermore, the use of a high molecular weight polymer improves water drainage rates by as much as 200% or more. This provides a more homogenous distribution of added cement throughout the backfill and prevents or minimizes segregation of the coarse and fine tailing particles. Thus, a set fill having a uniform particle size distribution is provided by the process of this invention whereby water-reactive setting material is retained with the solids without employing large excesses of setting material. Also the use of a watersoluble high molecular weight polymer improves the stabilization of the backfill mass, increases the strength and load bearing capacity of the set backfill, decreases clogging and blockage of mine drainage areas and excessive abrasion of ore haulage equipment, lowers maintenance costs for clean-up operations, facilitates mining ,operations, eliminates dilution of valuable ores and conserves such ores.

It is not necessary to employ ore tailings as the fill material since other fills can provide satisfactory strength when set. However, the use of ore tailings as fill is economically attractive since they are produced continuously in ore processing. In addition, ore tailings stored above ground in open areas frequently cause serious air aid water pollution, when spread by winds or rains. The present invention provides a very convenient method for disposing of such tailings, thus eliminating a storage problem. In the process of this invention, the ore tailings may or may not be deslimed prior to mixing with cement and water soluble high molecular weight polymer as fines retention agent.

Within the scope of the present invention, the backfill material contains a water-reactive setting material as for example, finely divided Portland cement, in an amount sufiicient to harden the backfill material and form a solid, strong mass within the earth cavity. The setting agent can be present in the fill in an amount between about 1:60 and about 1:10 weight ratio of setting agent to fill based upon the Weight of particulate solids in the fill. Amounts of setting agent above 1:10 weight ratio, although providing strong solid fill for earth cavities, are generally not required since the added strength obtained thereby does not economically justify the added cost of the setting agent.

The water-soluble high molecular Weight polymeric fines retaining agent is present in amounts to fix the fines substantially completely and relatively quickly within the backfill mass, thus providing uniform distribution of the coarse and fine components throughout the mass and permitting water containing very little or no solids material to drain rapidly from the mass. The polymer, usually added as a water solution 0.05 to 5% in concentration, is employed in amounts of about 0.001 to 2.0 pound per ton, preferably between about 0.005 and about 0.25 pound per ton based upon the weight of the solid portion of the fill material. The use of polymer in amounts of above about 2.0 lb./ton does not materially improve the retention of the fines in the mass and therefore is not economical.

The amount of water employed in the fill material is minimized to promote relatively quick drainage. However, sufficient Water, either present or added, is employed to promote homogeneous mixing of the solid material and to support reaction of the Water-reactive setting agents. Water is employed in amounts sufiicient to facilitate placement of the resultant mixture into the cavity. Fill mixtures as high as 50 to about 75% solids are employed frequently, 60-65% solids being preferred.

Representative anionic, cationic and nonionic Watersoluble polymers which can be employed as fines retention agents include both synthetic and naturally occurring high molecular weight hydrophilic polymeric materials.

Suitable synthetic anionic and nonionic polymers for use in the present invention are the polymers of methacrylic acid derivatives and copolymers of acrylic acid, for example, obtained from acrylic acid, the alkali metal and ammonium salts of acrylic acid, methacrylic acid, the alkali metal and ammonium salts of methacrylic acid, acrylamide, methacrylamide and the N-alkyl substituted amides. These polymeric compositions may be copolymers with other copolymerizing monomers, such as ethylene, propylene, isobutylene, styrene, alpha-methylstyrene, vinyl acetate, vinyl formate, vinyl ether, acrylonitrile, methacrylonitrile, vinyl chloride, vinylidene chloride, the alkyl acrylates, the alkyl methacrylates, the alkyl maleates, and the alkyl fumarates, and other olefinic monomers copolymerizable therewith. The copolymers of this type, having at least 50 mole percent of the acrylic or methacrylic acid derivatives, are preferred, and especially when the comonomer is hydrophobic or has no ionizable groups. Polymers of this type may be prepared directly by the polymerization of suitable monomers, or by the after chemical reaction of other polymers, for example by the hydrolysis of acrylonitrile or methacrylonitrile polymers, polyacrylamides and the like.

In connection with the various types of polyelectrolytic polymers suitable for the practice of this invention, the hydrophilicpolymer may be prepared directly by the polymerization or copolymerization of one or more of the various available organic monomers with aliphatic unsaturation, if the said compounds contain a hydrophilic group, for example, carboxyl groups. Generally, more types of polyelectrolyte polymers can be prepared by subsequent reactions of polymers and copolymers. For example, polymers containing nitrile groups may be hydrolyzed to form water-soluble amide and carboxy containing polymers. Similarly, copolymers of maleic anhydride and vinyl acetate may be hydrolyzed to form polymers containing hydrophilic lactone rings. Other hydrophilic polymers may be prepared by the hydrolysis of copolymers of vinyl acetate wherein the acetyl groups are removed, leaving hydroxy groups which promote the solubilization effect of polyelectrolytic groups present. By other reactions, non-hydrophilic polymers may be converted into lactam or amide containing polymers which are more hydrophilic. Polyvinyl alcohol, not in itself a polyelectrolyte, may be converted into polyelectrolytes by esterification with dibasic acids, one of said carboxylic acid groups reacting with the alcohol radical and the other providing the hydrophilic characteristics by a carboxy group on the side chain. Other soluble polymers may be prepared by the ammonolysis of ketone containing polymers, for example, polyvinyl methyl ketone. Similarly active halogen atoms may be reacted with bisulfite to substitute sulfonic acid group for the reactive halogens.

Thus, the various polyelectrolytes of the types described above are ethylenic polymers having numerous side chains distributed along a substantially linear continuous carbon atom molecule. The side chains may be hydrocarbon groups, carboxylic acid groups or derivatives thereof, phosphoric acid or derivatives thereof, alkoxy radicals and other organic groups, the number and the relative proportions of hydrophilic and hydrophobic groups being such as to provide a water-soluble polymeric compound having a substantially large number of ionizable radicals. The length of the said continuous carbon chain must be such as to provide compounds having a weight average molecular weight of at least 100,000.

Among the various polymers as described above, and water-soluble salts thereof, useful in the practice of the present invention, there may be mentioned polyacrylamide, polyacrylic acid, hydrolyzed polyacrylonitrile and polyacrylamide, sulfonated polystyrene, poly (ethylene oxides), acrylamide-acrylic acid copolymers, hydrolyzed styrene-maleic anhydride copolymer, acrylamide-acrylonitrile copolymer, methacrylic acid, hydrolyzed methacrylic acid-acrylonitrile copolymer, vinyl acetate-maleic anhydride copolymer, vinyl formate-maleic anhydride copolymer, vinyl methyl ether-maleic anhydride copolymer, isobutylene-maleic anhydride copolymer, styrene-maleic anhydride copolymer, ethyl acrylate-maleic anhydride copolymer, vinyl chloride-maleic anhydride copolymer, hydrolyzed acrylonitrile, ethylene-maleic anhydride adducts, vinyl acetate copolymer, hydrolyzed acrylonitrile-methacrylonitrile copolymer, hydrolyzed acrylonitrile-methacrylonitrile-vinyl acetate terpolymer, hydrolyzed acrylonitrile-methacrylic acid copolymer.

Such polymers having molecular weights as low as 100,000 are useful as are polymers having molecular weights which range over 2 to 15 million.

Suitable naturally occurring anionic or nonionic polymers or their derivates which can be employed herein as fines retention agents include the poly-aliphatic-hydroxy polymers, polysaccharides, starches, or carbohydrate polymers and their water-suspensible derivates such as manno-galactan, glucomman, hydroxyethylcellulose, extract of jack-in-the-pulpit bulb and green seeds, extract of calla lily bulb, extract of iris bulb and guns such as guar gum.

The manno-galactans are presently commercially derived from the endosperm of the seeds of ceretonia, siliqua, Cyamopsis tetragonoloba. They are also derivable from the endosperm of other leguminous seeds, e.g., those of Gleditsia triancanthos and Gymnocladum dioicous. Chemically, they are highly polymerized sugar complexes consisting of the two sugars, mannose and galactose in proportions which vary somewhat with the source. They are available as powdered products generally resembling starch in appearance. Polymerized sugar complexes which are classified as mannogalactans and which can be made water-soluble are also suitable for the practice of this invention.

Suitable cationic polymers which can be employed in the present invention include water-soluble quarternary ammonium salts resulting from the reaction between polyvinyl pyridine and butyl bromide and from the reaction of a primary diamine such as propylene diamine and alkyl halides such as 1,4-dichlorobutane and bromobutane, reaction products of alkylene polyamines with halohydrins; salts of polyvinylamine such as the acetate; the watersoluble formaldehyde reaction product of guanidine carbonate, or of certain substituted guanidine salts such as amino guanidine carbonate, guanidine-urea-formaldehyde condensates, polyethylene imine and salts such as the acetate or sulfate; salts of long chain polymeric secondary amines such as are formed by the reaction of 1,4-dibromobutane and 1,6-hexamethylenediamine; copolymers of acrylamide and diallyl dirnethyl ammonium halides, reaction products of polyacrylamide or polyacrylonitrile with ethylene diamine, reaction products ofrnethyl amine and epichlorohydrin and adipic acid, polyalkylene polyamines and epichlorohydrin and other cationic polymers. Such polymers of molecular weights as low as about 100,- 000 are useful in the process of this invention. Polymers of molecular weights of 0.5 to 2 million are useful.

The above types of polymers may be used singly or in combination to serve as fine retention agents for various types of backfill.

While it is not an intention to be limited to any one theory for explaining how the added polymers act to improve the characteristics of the backfill mass, a possible explanation for their action in retaining the fines in the mass is that the polymers bond the fines, particularly the added fine setting agents, to the coarser mineral portions of the mass. The fines are thus fixed within the mass, permitting the water to drain freely from the mass without exudation of large quantities of the fines.

The following examples illustrate the process of the present invention and are not intended to limit the same. Quantities of materials used are in terms of parts by weight unless otherwise specified.

EXAMPLE 1 To deslimed ore flotation tailings obtained in the flotation of a copper-nickel ore, a typical back-fill material, 1 part of cement in powder form for each 30 parts of the tailings was added and mixed thoroughly to form a suspension of about 62% solids. Polyacrylamide of a molecular weight in the range of -12 million was added as a fines retention agent to representative samples of the resulting mixture in quantities of 0.0025 to 0.04 pound per ton of dry tailings. A 2640-g. sample of each mixture then was allowed to drain into a cylinder which was 3.5 inches in diameter and 16 inches in height and was closed at the bottom by means of a perforated metal disc covered with burlap such as is used as a retainer in mine backfilling operations. This arrangement permitted free drainage of water at the base of the cylinder and permitted the measurement of the volume and solids content of such drainage over a timed interval.

The samples of backfill in the cylinder were permitted to drain until the layer of water at the surface had disappeared. The time required for this drainage, the rate of drainage of water through the backfill column, and the solids content of the drainage were determined and are given in the following Table I.

TABLE I Time to Cement to Polyacryl- Water Drain- Solids in Remove Sur- Backfillamide age Rate Drainage face Water Weight Ratlo (113 ./ton) (gaL/hour/ton) (lb ./ton) (minutes) The action of the polyacrylamide as a fines retention agent and for improving the rate of water drainage from the backfill mass is demonstrated clearly by the above data.

The added polyacrylamide did not adversely effect the compression of the backfill, since the mixtures containing cement both with and without this fines retention agent occupied the same volume and measured the same height, 7 inches, in the cylinder.

The above cylinders were sectionalized for examination. The cylinders containing cement but no polyacrylamide showed a segregation of mineral and cement at the top portions of each column. The addition of polyacrylamide eliminated this segregation and provided a uniform distribution of mineral and cement fines throughout the entire backfill column.

EXAMPLE 2 The effect of variations in the cement to backfill ratio in the presence of a fines retention agent is shown in the following tests on 2640-gram lots of deslimed flotation tailings from the processing of a copper ore. The samples were treated with the polyacrylamide and by the method of Example 1. The cement to dry backfill ratio was varied from 1:60 to 1:12. Polyacrylamide was used in all tests in the amount of 0.01 lb./ ton. Control tests without polymer were conducted. The results of these tests are summarized in the following Table II.

TABLE II Water Drain- Cernent to Backfill Polyacrylage Rate, gaL/ Solids in Drain- Werght Ratio amide, 1b./ton hour/ton age, lb./ton

The data set forth in Table II show the polyacrylamide is eifective over a wide range of cement to backfill ratios.

EXAMPLE 3 In a surface mixing plant, one part of Portland cement was added to thirty parts of a deslimed flotation tailing from the treatment of a second copper ore. The mixture, 63.6% solids, was pumped 2000 feet below the surface to a mined-out stope in which two equal areas had been partitioned ofl with burlap-lined, wooden frames for testing purposes. Just before place-ment in the first test area, a ZOO-ton lot of the mixture was treated with 0.0073 lb./ton of the polyacrylamide of Example 1. A second 200-ton lot of backfill, identical with the first but with no polyacrylamide, was pumped into the second test area. Two minutes after the placement of each lot of backfill had been completed, a sample of the water drainage from backfill mass was taken upon each test 7 area for a determination of the suspended solids con-- tent. The results of these tests are summarized in the following Table III.

I claim: 1. A process for filling a subterranean cavity which comprises placing in said cavity a water slurry comprised of (a) particulate mineral solids ('b) a water-reactive set- TABLE III ting agent and (c) a water-soluble high molecular weight N Pdyficrylamid polymeric fines retention agent, and permitting the water yacrylamide Added g to drain from the cavity, whereby the remainder of the 211 2 ;aggf z igggzfg fg emplaced material subsequently sets to a hardened co- Baekfill Pile (inches) 12 0.375 hesive mass.

2. The process of claim 1 wherein the amount of set- A reduction in the slime content in the drainage water ting agent employed is in the ratio of about 1:10 and of higher than 95% in addition to a marked decrease in about 1:60 based upon the weight of particulate solid the slime layer at the surface of the backfill in place is material. shown in the above comparative figures. The polyacryl- 3. The process of claim 1 wherein the polymeric fines amide-treated backfill which remained after the water 15 retention agent is a polyacrylamide having a molecular had drained had a more homogeneous distribution of weight above about 100,000. coarse and fine material while the cement was more 4. The process of claim 1 wherein the polymeric fines homogeneously distributed throughout the backfill mass. retention agent is a hydrolyzed polyacrylamide having a The drained backfill was set to form a strong support for molecular weight above about 100,000. the mined out area which permitted mining the ore in 5. The process of claim 1 wherein the fines retention locations adjacent to the previously mined out area. agent is a copolymer of acrylamide and acrylic acid having a molecular Weight above about 100,000. EXAMPLE 4 6. The process of claim 1 wherein the fines retention Var s a kfi l materials Were mixed with cement and agent is a hydrolyzed polyacrylonitrile having a molecular then treated with various polymers as fines retention agents weight above abo t 100,000. by the method described in Examp e 1. The polymers use 7. The process of claim 1 wherein the mineral solids were from synthetic and natural sources and were nonare ore tailings. ionic, anionic, and cationic in type. The results of these 8. The process of claim 2 wherein the solid material is tests are summarized in the following Table IV and clearly ore tailings. demonstrate the generalapplicability of the priciples of 9. The process of claim 1 wherein the solid material is this invention in that the compositions used as fines reore tailings. tention agents improved water drainage rates and de- 10. The process of claim 1 wherein said setting agent creased the drainage of fine solids from the backfill mass. is portland cement.

TABLE IV Drainage Rate Percent Solids Cegengfitlrl) Fines Retention Agent gal. [hour/ton in Drainage ac Backfill Material Weight Ratio Type lb./ton Control 1 Treated Control 1 Treated Dgglianed eogper-nickel flotation tailings, 1/30 Polyacrylamide, 2 mM. mol. wt 0.01 160 l. 61 0.92

. so 1 s.

a 1/30 Polyacrylamide, 10 mM. mol. wt 0.01 110 185 1. 61 0.81 Deslimed gold cyanidation tailings, 63.1% 1/30 Hydrolyzed (30%) polyacrylam de, 0.015 83 145 2. 54 0.93

solids. 10 mM. 111 w Copper flotation tailings (A), 62.5% solids.-- 1/30 Hydfiolyzed polyacrylonitrile, 0i25 0. 02 183 1. 86 0. 71

m m wt. Copper flotation tailings (B), 64.1% solids 1/30 70:30 Aerylamide-Acrylie acid 00- 0. 01 115 167 2.10 1. 03

polymer, 7 mM. mol. wt. Dtefilisraed pepper flotation tailings (C), 1/30 Guar gum 0. 02 123 172 3. 02 0.97

so 1 s. Lead-silver flotation tailings, 63.1% solids 1/35 Sulfonated polystyrene 0.009 117 168 2. 73 0. 86 Sand; 65 mesh, 65% solids 1/30 Polyethylenimine 0. 015 196 1. 52 0, 61 Crushed slag, 63.1% solids 1/25 Condefilsatehof methylamine and 0. 02 122 187 1. 72 0. 74 epic oro y Lead-zinc flotation tailings (A), 61.7% 1/30 Reaction product of polyaerylamide 0.20 128 185 2. 37 0. 69

solids. and ethylene diamine. Zine flotation tailings (A), 60.9% solids 1/35 Reaction product of polyacrylonitrile 0.020 218 2. 86 1. 01

and ethylenediamine.

Desllmed zinc flotation tailings (B), 61.7% 1/30 Condens1te oi adipic acid-tetraethyl- 0. 018 127 191 3.07 0. 32 s i s. ene polyamine-epichlorohydrin. Copper-zino-lead ore flotation tailings, 62.0% Polyacrylic acid, 6 mM. mol. Wt 0. 01 115 181 2. 86 0. 62

so Zinc flotation tailings (C), 62.5% solids 1/30 Polysaccharide prepared from glucose" 0.03 115 158 2. 76 1. 99

1 N o fines retention agent added. 3

References Cited UNITED STATES PATENTS 3,312,070 4/1967 Matsuo et al. 6136 3,340,693 9/1967 Row 6136 JACOB SHAPIRO, Primary Examiner 

